Monolithic widely-tunable coherent receiver

ABSTRACT

Various embodiments of a coherent receiver including a widely tunable local oscillator laser are described herein. In some embodiments, the coherent receiver can be integrated with waveguides, optical splitters and detectors to form a monolithic optical hetero/homodyne receiver. In some embodiments, the coherent receiver can demodulate the full phase information in two polarizations of a received optical signal over a range of optical wavelengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/789,344 filed on May 27, 2010 titled “Monolithic Widely-TunableCoherent Receiver,” which claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Application 61/182,017 filed on May 28, 2009 titled“Chip-Based Advanced Modulation Format Transmitter,” and claims thebenefit under 35 U.S.C. §119(e) of U.S. Provisional Application61/182,022 filed on May 28, 2009 titled “Monolithic Widely-TunableCoherent Receiver.” Each of the above-identified applications isincorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

Various embodiments of the invention relates to the area of opticalcommunications photonic integrated circuits (PICs). In particular, theembodiments described herein generally relate to receivers for fiberoptic or free space communications, and coherent receivers withmonolithically integrated tunable local oscillator light sources.

2. Description of the Related Art

High bandwidth time division multiplexing (TDM) links can advantageouslyreduce the size, weight and/or power required in the system. Receiverarchitectures are configured to receive and detect time divisionmultiplexed signals are generally designed to operate at a certainbit-rate. It may be desirable to design TDM links and systems thatoperate at variable bit-rates. In such systems, it may not be possibleto use current receiver architectures since it may not be possible torapidly configure the current coherent receiver devices to detectsignals having variable operational bit-rates. Thus, there is a need fora scalable coherent receiver that can switch rapidly to detect differentoperational bit-rates. For some applications it may be advantageous tohave shot noise limited receivers.

SUMMARY

Systems and methods that enable coherent receivers with monolithicallyintegrated tunable local oscillator light that can switch rapidly todetect different operational bit-rates may be beneficial in opticalcommunication networks and systems. Example embodiments described hereinhave several features, no single one of which is indispensible or solelyresponsible for their desirable attributes. Without limiting the scopeof the claims, some of the advantageous features will now be summarized.

Various embodiments of the devices described herein provide amonolithically integrated semiconductor optical coherent receiver with amonolithically integrated widely tunable local oscillator that can tuneto any transmission wavelength in a given range. In some embodiments,the widely tunable laser oscillator can be configured to tune to anytransmission wavelength in a given range, wherein the range may belarger than the range that can be achieved by refractive index tuning ofthe semiconductor material alone. In some embodiments, the tuning rangemay be greater than approximately 15 nm. In certain embodiments, thetuning range may be approximately 40 nm to 100 nm. In some embodiments,the tuning range may be approximately 20 nm, approximately 25 nm,approximately 30 nm, approximately 35 nm, approximately 40 nm,approximately 45 nm, approximately 50 nm, approximately 55 nm,approximately 60 nm, approximately 65 nm, approximately 70 nm,approximately 75 nm, approximately 80 nm, approximately 85 nm,approximately 90 nm, or approximately 95 nm. In various embodiments, thetuning range can be between any of the values listed above. In someembodiments, the tuning range may be less than approximately 15 nm orgreater than approximately 100 nm.

In some embodiments, the coherent receiver can comprise at least twobalanced photo detector pairs or four balanced pairs for Quadrature andin-phase inputs, thereby allowing for coherent optical demodulationusing an on-chip, fully integrated polarization diversity configuration.In some embodiments, the coherent receiver can include a monolithicallyintegrated polarization beam splitter and/or a polarization-rotator onthe same die. Such a configuration may advantageously allow for a singleinput implementation of the coherent receiver and for the adjustment ofthe received polarization.

Various embodiments described herein include a compact optical coherentreceiver having a reduced die size. For example, the die size of thevarious embodiments of the optical receiver device described herein canbe between approximately 0.5 square mm and approximately 3 square mm. Invarious embodiments, the die size of the optical receiver device can bebetween approximately 1.5 square mm and approximately 2.5 square mm. Invarious embodiments, the die comprises a monolithically integratedoptical receiver device that is included in packaging to form thedevice. In various embodiments, the die can comprise a monolithicallyintegrated optical receiver device that will be coupled to opticalfibers or RF/electrical connectors. The decrease in the footprint and/orthe die size of the integrated optical receiver device canadvantageously reduce fabrication complexity required to integrate asingle surface-ridge waveguide structure and improve yield.

In various embodiments described herein, an optical receiver comprisinga widely tunable laser, one or more photo detectors and a polarizationrotator may be monolithically integrated on a single die having a commonsubstrate is disclosed. In various embodiments, monolithic commonsubstrate integration can include processes and techniques that placeall the subcomponents of the device on a common substrate throughsemiconductor device processing techniques (e.g. deposition, epitaxialgrowth, wafer bonding, wafer fusion, etc). In some embodiments, theoptical receiver comprising a widely tunable laser, one or more photodetectors and a polarization rotator may be integrated on a single diehaving a common substrate, through other techniques such as flip-chipbonding, etc. Monolithic common substrate integration can provide areduction in device insertion losses. Such tunable optical receiverdevices can be advantageous in reducing the number of components anddevices required in an optical system. Other advantages of an integratedtunable optical receiver can be compact die size, reduced footprint,faster tuning mechanisms, and the lack of moving parts—which can bedesirable for applications subject to shock, vibration or temperaturevariation. Integrating an optical receiver on a single die can offerseveral other advantages as well, such as precise phase control,improved performance and stability of the receiver, and compactimplementation. Some additional benefits of integrating a tunable laserwith a photo detector on a single die can be: the ability to adjust oroptimize the device performance.

Various embodiments, described herein include a complex optical receiverfabricated on a small die size. Such devices can be fabricated usingrelatively simple fabrication techniques and/or integration platforms.In various embodiments described herein, optical interconnect losses canbe reduced by reducing interconnect length rather than by includingcomplex low-loss optical waveguide structures.

Various embodiments of the optical receiver described herein comprise acommon substrate comprising a III-V material such as Indium Phosphideand one or more epitaxial layers (InP, InGaAs, InGaAsP, InGaP, InAlGaAsetc.); a laser resonator, formed on the common substrate in theepitaxial structure; and a plurality of photo detectors formed on thecommon substrate. In various embodiments, the sub-components of theoptical receiver such as waveguides, photonic components, splitters,etc. can be formed in the same epitaxial structure as the epitaxialstructure in which the laser is formed. In some embodiments thecomponents of the optical receiver such as waveguides, photoniccomponents, splitters, etc. can be formed in one or more epitaxialstructures that are different from the epitaxial structure in which thelaser is formed.

In various embodiments a monolithically integrated optical receiver dieis described. The monolithically integrated optical receiver die cancomprise an input interface and at least one monocrystalline substrate.The optical receiver die can further comprise a tunable laser resonatormonolithically integrated with the substrate, the tunable laserresonator comprising an output reflector and a tuning section, thetunable laser resonator configured to emit optical radiation from theoutput reflector along an optical axis, such that the wavelength of theemitted optical radiation is tunable over a wide wavelength range frombetween about 15 nm to about 100 nm, wherein the wide wavelength rangeis represented by Δλ/λ and is configured to be greater than a ratioΔn/n, wherein λ, represents the wavelength of the optical radiation, Δλrepresents the change in the wavelength of the optical radiation, nrepresents the refractive index of the tuning section, and Δn representsthe change in the refractive index of the tuning section. Themonolithically integrated optical receiver die also comprises a firstoptical mixer monolithically integrated with the substrate such that thefirst optical mixer is disposed at a distance less than approximately750 μm from the input interface as measured along the optical axis, saidfirst optical mixer having at least a first and a second input waveguideand a plurality of output waveguides, said first input waveguide of thefirst optical mixer optically connected to the laser resonator, saidsecond input waveguide of the first optical mixer configured to receivea modulated optical signal from the input interface. The monolithicallyintegrated optical receiver die further comprises a second optical mixermonolithically integrated with the substrate such that the secondoptical mixer is disposed at a distance less than approximately 750 μmfrom the input interface as measured along the optical axis, said secondoptical mixer having at least a first and a second input waveguide and aplurality of output waveguides, said first input waveguide of the secondoptical mixer optically connected to the laser resonator, said secondinput waveguide of the second optical mixer configured to receive amodulated optical signal from the input interface. In variousembodiments, a polarization rotator can be monolithically integratedwith substrate, said polarization rotator arranged at an angle betweenabout 20 deg and 160 deg or between about −20 deg and −160 deg withrespect to the optical axis. In various embodiments, the polarizationrotator may be disposed at an angle θ between about 20 degrees and 160degrees or between about −20 degrees and −160 degrees with respect tothe crystallographic axis of the monocrystalline substrate. A pluralityof photo detectors can be further monolithically integrated withsubstrate, each of the plurality of photo detectors being opticallyconnected to one of the plurality of output waveguides of the first orthe second optical mixer.

In various embodiments a monolithic tunable polarization rotator isdescribed herein. The monolithic tunable polarization rotator cancomprise a monocrystalline substrate; an optical splitter comprising aninput waveguide and at least two output waveguides; a pluralityelectrodes disposed on each the at least two output waveguides, of theoptical splitter, said plurality of electrodes configured to control anamplitude and a phase of the electromagnetic radiation propagating inthe at least two output waveguides; and an optical coupler comprising atleast one output waveguide, said optical coupler configured to receiveinput from the at least two output waveguides of the optical splitter.

In various embodiments, a method of manufacturing a monolithicallyintegrated optical receiver die is described. The method comprisesproviding an input interface; and providing at least one monocrystallinesubstrate. The method further comprises monolithically integrating atunable laser resonator with the substrate, said tunable laser resonatorcomprising an output reflector and a tuning section, said tunable laserresonator configured to emit optical radiation from the output reflectoralong an optical axis, such that the wavelength of the emitted opticalradiation is tunable over a wide wavelength range from between about 15nm to about 100 nm, wherein the wide wavelength range is represented byΔλ/λ and is configured to be greater than a ratio Δn/n, wherein λrepresents the wavelength of the optical radiation, Δλ represents thechange in the wavelength of the optical radiation, n represents therefractive index of the tuning section, and Δn represents the change inthe refractive index of the tuning section. The method also comprisesmonolithically integrating a first optical mixer with the substrate suchthat the first optical mixer is disposed at a distance less thanapproximately 750 μm from the input interface as measured along theoptical axis, said first optical mixer having at least a first and asecond input waveguide and a plurality of output waveguides, said firstinput waveguide of the first optical mixer optically connected to thelaser resonator, said second input waveguide of the first optical mixerconfigured to receive a modulated optical signal from the inputinterface. In the method of manufacturing described herein a secondoptical mixer can be monolithically integrated with the substrate suchthat the second optical mixer is disposed at a distance less thanapproximately 750 μm from the input interface as measured along theoptical axis, said second optical mixer having at least a first and asecond input waveguide and a plurality of output waveguides, said firstinput waveguide of the second optical mixer optically connected to thelaser resonator, said second input waveguide of the second optical mixerconfigured to receive a modulated optical signal from the inputinterface. The method further comprises monolithically integrating apolarization rotator with substrate, said polarization rotator arrangedat an angle between about 20 deg and 160 deg or between about −20 degand −160 deg with respect to the optical axis; and monolithicallyintegrating a plurality of photo detectors with substrate, each of theplurality of photo detectors being optically connected to one of theplurality of output waveguides of the first or the second optical mixer.In various embodiments, the polarization rotator may be disposed at anangle θ between about 20 degrees and 160 degrees or between about −20degrees and −160 degrees with respect to the crystallographic axis ofthe monocrystalline substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an embodiment of a monolithicallyintegrated receiver comprising a tunable laser and an optical mixer.

FIGS. 2A-2C schematically illustrate various embodiments of amonolithically integrated optical polarization rotator.

FIG. 3 schematically illustrates an embodiment of a folded balancedreceiver including a tunable laser, a 2×2 optical coupler and a photodetector.

FIG. 4 schematically illustrates a folded I/Q receiver including atunable laser, a 2×4 optical hybrid coupler and two sets of balanced (Iand Q) receivers.

FIG. 5 schematically illustrates a folded I/Q receiver including atunable laser, a tunable polarization rotator, 2×4 optical hybridcoupler and two sets of balanced (I and Q) receivers.

FIG. 6 schematically illustrates a folded polarization diverse I/Qreceiver including a tunable laser, 1×2 input light splitter,polarization rotation elements, and two sets of full I/Q receivers, onefor each polarization.

FIG. 7 schematically illustrates an optical receiver configurationincluding optical carrier recovery function.

These and other features will now be described with reference to thedrawings summarized above. The drawings and the associated descriptionsare provided to illustrate embodiments and not to limit the scope of thedisclosure or claims. Throughout the drawings, reference numbers may bereused to indicate correspondence between referenced elements. Inaddition, where applicable, the first one or two digits of a referencenumeral for an element can frequently indicate the figure number inwhich the element first appears.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although certain preferred embodiments and examples are disclosed below,inventive subject matter extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and tomodifications and equivalents thereof. Thus, the scope of the claimsappended hereto is not limited by any of the particular embodimentsdescribed below. For example, in any method or process disclosed herein,the acts or operations of the method or process may be performed in anysuitable sequence and are not necessarily limited to any particulardisclosed sequence. Various operations may be described as multiplediscrete operations in turn, in a manner that may be helpful inunderstanding certain embodiments; however, the order of descriptionshould not be construed to imply that these operations are orderdependent. Additionally, the structures, systems, and/or devicesdescribed herein may be embodied using a variety of techniques includingtechniques that may not be described herein but are known to a personhaving ordinary skill in the art. For purposes of comparing variousembodiments, certain aspects and advantages of these embodiments aredescribed. Not necessarily all such aspects or advantages are achievedby any particular embodiment. Thus, for example, various embodiments maybe carried out in a manner that achieves or optimizes one advantage orgroup of advantages as taught herein without necessarily achieving otheraspects or advantages as may also be taught or suggested herein. It willbe understood that when an element or component is referred to herein asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent therebetween.

FIG. 1 illustrates an embodiment of a receiver comprising a tunablelaser and a photo detector. The embodiment illustrated in FIG. 1,comprises a common substrate 101. In some embodiments, the receiver cancomprise one or more epitaxial structures formed on the commonsubstrate. In various embodiments, the one or more epitaxial structurescan include layers or stacks of layers grown, deposited or formed on thecommon substrate such that the one or more layers have a latticestructure and orientation substantially similar to the common substrate.The receiver can further comprise a first optical waveguide 102configured to receive an external optical input signal that isintegrally disposed on the common substrate; a widely tunable localoscillator/laser resonator 104 integrally disposed on the commonsubstrate in the same or different epitaxial structure as the firstoptical waveguide 102; a second optical waveguide 103 disposed in thesame epitaxial structure as the local oscillator 104 and opticallycoupled to the local oscillator 104; an optical mixer (or coupler)structure 105 disposed on the common substrate and disposed external tothe local oscillator 104; and at least one photo detector 107,integrally disposed on the common substrate. In various embodiments, thereceiver die can comprise an input interface 110 through which theexternal modulated optical signal is coupled into the receiver die. Invarious embodiments, an optional mode converter can be integrated at aninput edge of the first optical waveguide 102 to improve the couplingefficiency of the external modulated optical signal. In variousembodiments, the input interface 110 can comprise an input facet of thereceiver die. In some embodiments, the input interface 110 may comprisethe mode converter. In various embodiments, the optical mixer 105 may bedisposed at a distance of approximately 750 μm from the input interfaceas measured along the optical axis. In various embodiments, the opticalmixer 105 can be disposed at a distance of approximately 750 microns orless from the input interface as measured along a horizontal directionparallel to a first edge of the die (e.g. along the y-axis). In variousembodiments, the optical mixer structure 105 is configured to mix (orcombine) the optical signal from the second optical waveguide 103 withthe external input optical signal from the first optical waveguide 102.The optical mixer structure 105 can include at least one outputwaveguide 106 which is optically connected to the photo detector 107.The photo detector can be configured to detect the combined outputsignal from the optical mixer 105. In various embodiments, the photodetector can comprise a photodiode. Other photo sensors can also be usedin various embodiments. These and other features are further describedbelow.

Monocrystalline Substrate

In various embodiments, the monocrystalline substrate 101 may compriseone or more epitaxial structures. In various embodiments, an epitaxialstructure may be formed by a method of depositing a monocrystalline filmon a monocrystalline substrate. In various embodiments, epitaxial filmsmay be grown from gaseous or liquid precursors. Because the substrateacts as a seed crystal, the deposited film takes on a lattice structureand orientation identical to those of the substrate. In variousembodiments, the epitaxial structure comprises InGaAsP/InGaAs orInAlGaAs layers on either a GaAs or InP substrate grown with techniquessuch as MOCVD or Molecular Beam Epitaxy (MBE) or with wafer fusion of anactive III-V material to a silicon-on-insulator (SOI) material.

Laser Resonator

As discussed above in various embodiments, the laser resonator 104 maybe formed on the common substrate and/or in one or more epitaxialstructures formed on the common substrate. In various embodiments, thelaser resonator 102 can include a widely tunable laser. As discussedabove, in various embodiments, the one or more epitaxial structures caninclude layers or stacks of layers grown, deposited or formed on thecommon substrate such that the one or more layers have a latticestructure and orientation substantially similar to the common substrate.In various embodiments, the widely tunable laser can comprise a lasingcavity disposed between two mirrors or reflectors and a tuning section.The optical radiation or laser light generated by the widely tunablelaser is output from the reflector disposed closer to the output side ofthe laser cavity (output reflector) along an optical axis. In variousembodiments of the optical transmitter device the optical axis can bealigned parallel to the crystallographic axis of the monocrystallinesubstrate 101 (e.g. 011 axis for an InP substrate). In the embodimentillustrated in FIG. 1, the optical axis can be aligned parallel to the+y axis.

In various embodiments, the wavelength of the optical radiation emittedfrom the widely tunable laser can be tuned over a wide wavelength rangefrom between about 15 nm to about 100 nm. Without subscribing to anyparticular theory, in various embodiments, the widely tunable laser canhave a relative wavelength change (Δλ/λ) that is larger than theavailable relative index tuning (λn/n) inside the laser cavity, whereinλ represents the wavelength of the optical radiation, Δλ represents thechange in the wavelength of the optical radiation, n represents therefractive index of the tuning section, and Δn represents the change inthe refractive index of the tuning section. The widely tunable laseroscillator can be configured to tune to any transmission wavelength in agiven range, wherein the range may be larger than the range that can beachieved by refractive index tuning of the semiconductor material and/orthe tuning section alone. Without subscribing to any particular theory,the wide wavelength tuning in some embodiments of the widely tunablelaser can be achieved by using the Vernier effect, in which the twomirrors or reflectors defining the lasing cavity have multiplereflection peaks. The lasing wavelength is then defined by the overlapbetween one reflection peak of each mirror. Tuning the index in one ofthe mirrors or the tuning section (e.g. by applying a voltage toelectrodes disposed on the mirrors and/or the tuning section) can shiftthe wavelength of each of the many reflections, causing a different pairof reflection peaks to come into alignment, thus shifting the lasingwavelength further than that of the wavelength shift of the tunedmirror.

In various embodiments, the widely tunable laser as described herein canhave a tuning range from about 15 nm to about 100 nm around 1550 nm. Insome embodiments, the laser resonator 104 can have a tuning range thatis greater than approximately 15 nm. In certain embodiments, the tuningrange may be approximately 40 nm to 80 nm. In some embodiments, thetuning range may be approximately 20 nm, approximately 25 nm,approximately 30 nm, approximately 35 nm, approximately 40 nm,approximately 45 nm, approximately 50 nm, approximately 55 nm,approximately 60 nm, approximately 65 nm, approximately 70 nm,approximately 75 nm, approximately 80 nm, approximately 85 nm,approximately 90 nm, or approximately 95 nm. In certain embodiments, thetuning range may have a value between any of the values provided above.In some embodiments, the tuning range may be less than approximately 15nm or greater than approximately 100 nm.

In various embodiments, the laser oscillator can include any of avariety of widely tunable lasers such as, for example, Sampled GratingDistributed Bragg Reflector (SGDBR) lasers, Superstructure gratingDistributed Bragg reflector, Digital Supermode Distributed BraggReflector (DSDBR), Y-branch or folded tunable laser, etc.

Optical Mixer

In various embodiments, the optical mixer 105 can comprise withoutlimitation a multimode interference coupler, evanescent coupled-modecoupler, reflection coupler, or Y-branch coupler, and can have at least2 input waveguides and one or more output waveguides 106. One functionof the mixer 105 can be to mix the light generated from the laseroscillator 104 and the external optical signal coupled into the firstoptical waveguide 102. The two or more signals can be coupled in equalor unequal ratios. To simplify control of the coupling ratio, in variousembodiments, an electrode can be integrated with the mixer 105. Onepurpose of the electrode can be to change the optical index in the mixer105 that can change the coupling ratio. In various embodiments, themixer 105 can be designed to reduce or prevent reflections of the laser104 back into the laser cavity by ensuring that the input waveguides ofthe mixer 105 are arranged at an obtuse angle with respect to the outputwaveguides 106.

Photo Detector

As discussed above in various embodiments, the photo detector 107 can beintegrated with the common substrate 101, in the same or differentepitaxial structure as other components. In various embodiments, thephoto detector 107 is disposed at one end of the output waveguide 106.In some embodiments, the waveguide 106 and the photo detector 107 can beterminated in such a way as to reduce or minimize the reflection backinto the laser oscillator 104. In some embodiments, the waveguide 106and the photo detector 107 can be arranged such that any reflectedoptical radiation is dispersed into the substrate. In some embodimentsthe waveguide can 106 can comprise a very low reflection angled, taperedwaveguide.

In some embodiments, the optical mixer 105 can comprise two or moreoutput waveguides. Photo detectors can be located along each outputwaveguide. The two photo detectors can be interconnected in such a waythat only the differential electrical signals may be added, and thecommon signals for both detectors can be subtracted and cancelled out.This can be achieved either on-chip, by connecting the two detectors inseries, or off-chip, where a differential amplifier selectivelyamplifies the difference in photocurrent between the two photodetectors.

Other Optical/Photonic Components

In various embodiments, light can be coupled into the optical receiverdie using an optical fiber or a lens system through an input interface110. In some embodiments, the input interface 110 can include the inputfacets on an input side of the optical receiver. In some embodiments, amode-converter can be integrated with the first optical waveguide 102.In various embodiments, the mode converter can include a structure thatconverts the shape of the optical mode in the waveguide 102 to bettermatch that of input optical fiber or lens to improve coupling betweenthe external fiber and the substrate waveguide. A mode-converter may beformed using several methods and techniques. One approach can be totaper the optical waveguide such that the waveguide is transformed froma buried ridge structure to a surface ridge structure. Withoutsubscribing to any particular theory, a buried ridge structure caninclude a waveguide structure where the waveguide is clad with claddingmaterial other than air all around. Without subscribing to anyparticular theory, surface ridge can include a waveguide structure whereguiding is provided by index variation in the cladding region (includesfor example, between semiconductor and air).

In some embodiments, a semiconductor optical amplifier can be integrallydisposed on the common substrate 101 in the same or different epitaxialstructure. The semiconductor optical amplifier can be diposed along thefirst optical waveguide 102 and/or along the second optical waveguide103. The semiconductor optical amplifier can provide optical waveguidegain to compensate for optical coupling losses and optical propagationlosses in the first or second optical waveguides 102 and 103. Similarly,optical amplifiers can also be disposed in waveguide sections after themixer 105 along the output optical waveguide 106.

Optical Polarization Controlling Components

Several components of the receiver can be polarization sensitive. Forexample, in some embodiments, the optical mixer 105 may be of the typethat only creates coherent beam combination if the polarization of thetwo input signals is matched. Polarization beam splitters and rotatorscan be integrated on the common substrate 101 in the same or differentepitaxial structure as other components to generate polarizationindependent operation of the die.

In various embodiments, the polarization beam splitter elements may beformed on the common substrate 101, in the same or different epitaxialstructure as other components, with the purpose of splitting the inputcoupled light into the TE and TM polarized modes of the commonwaveguide. One embodiment of this element can be realized by using thephenomenon of birefringence between two modes. A polarization rotatingelement can convert TM polarized light into TE polarized light and viceversa.

A polarization rotator can be formed by inducing a birefringence in thewaveguide, for example, by fabricating an asymmetric waveguide. Anasymmetric waveguide can be fabricated by etching the sidewall of theoptical waveguide at an angle. By selecting the appropriate length ofthe angled etch, a halfwave plate can be formed that can rotate linearlypolarized TE or TM light by 90 degrees. Other approaches and designs canalso be used.

FIG. 2A schematically illustrates top view of an embodiment of apolarization rotator that can be integrated in the optical receiver.FIG. 2B schematically illustrates a cross-sectional view of thepolarization rotator illustrated in FIG. 2A along an axis 204 parallelto the normal to the substrate 203 (e.g. parallel to the z-axis). In oneembodiment, the polarization rotator comprises an asymmetric waveguideridge 202 that is disposed on a substrate 203. In various embodiments,the polarization rotator can be formed by modifying a waveguide section201 of the optical transmitter device using semiconductor deviceprocessing techniques. In various embodiments, the asymmetric waveguideridge 202 can be formed on a slab waveguide 205 which comprises ahigh-index material. In some embodiments, the waveguide ridge 202 canhave a first edge 202 a disposed at a first angle with respect to thenormal to the substrate and a second edge 202 b disposed at a secondangle with respect to the normal to the substrate. In variousembodiments, the first and the second angle can be different from eachother. In various embodiments, the first angle can be approximatelyparallel to the normal to the substrate as shown in FIG. 2B. Theasymmetric nature of waveguide ridge 202 results in a bi-refringentwaveguide structure.

As discussed above, the asymmetric waveguide structure 202 can be formedby using an etching process. For example, in one method of fabricatingthe polarization rotator, the asymmetric waveguide structure 202 is dryetched on one side of the waveguide ridge 202 to form the edge 202 a,and wet etched on the other side of the waveguide ridge 202 to form thesloping edge 202 b. In some embodiments, the method can include etchingthrough the slab waveguide 205. Etching through the slab waveguide 205can be advantageous to realize a polarization rotator structure withreduced footprint.

In one method of fabricating the polarization rotator on an InPsubstrate, the sloping edge 202 b can be formed by employing a wet etchat a waveguide section oriented around 90 degrees with respect to thelaser ridge—which gives around 40-50 degrees wet etch plane—that stopson the InGaAsP or InAlGaAs waveguide core (e.g. slab waveguide 205) or astop etch layer. If a different orientation is chosen for thepolarization rotator (e.g. perpendicular to or within 20-160 degrees or−20 to −160 degrees from the laser axis) the wet etch will align thewaveguide edge to an angle not perpendicular to the substrate. The abovedescribed method of fabricating the polarization rotator can be arepeatable process and can yield polarization rotators with a smallfootprint.

FIG. 2C illustrates an embodiment of a tunable polarization rotatorwhich can be integrated with the optical receiver. The tunablepolarization rotator comprises an input waveguide 210 that is connectedto an optical splitter 212 having with two output waveguides. Electrodes214 a, 214 b, 216 a and 216 b may be provided to the two outputwaveguides. In various embodiments, the optical splitter 212 can be apolarization beam splitter. A voltage between approximately 1V toapproximately −6V can be applied to the electrodes 214 a and/or 214 b tochange the transmitted optical intensity. An electric current in therange of approximately, 0 mA to approximately 15 mA may be provided tothe electrodes 216 a and/or 216 b to adjust the phase of the opticalradiation propagating through the output waveguides. In variousembodiments, the voltage and the current can be provided by using anexternal drive circuit. The tunable polarization rotator can furthercomprise a polarization rotator 218 that can be disposed in one of theoutput waveguides. The polarization rotator 218 can be similar to thevarious embodiments of the polarization rotators described above. Thetunable polarization rotator can further comprise an optical coupler 220having an output waveguide 222 and configured to combine the opticaloutputs of the two output waveguides of splitter 212. With theappropriate adjustment of optical phase and intensity, the polarizationstate of the signal in the output waveguide 222 can be tuned.

Some Preferred Embodiments

Some preferred embodiments are described below. It is understood thatthese represent a few possible embodiments out of a range of possibleembodiments that have some similarities to the embodiment illustrated inFIG. 1 and the sub-components described therein.

Embodiment 1

FIG. 3 illustrates an embodiment of a coherent receiver that is formedon a common substrate. In the illustrated embodiment, an externalmodulated optical signal is coupled into a first waveguide integrallyformed on the common substrate. Improved optical coupling efficiency tothe integrated receiver can be achieved through integration of amode-converter 301. In various embodiments, the receiver die maycomprise an input interface through which the external modulated opticalsignal is input to the receiver die and coupled into a waveguide suchthat the incoming external optical modulated optical signal propagatesalong a direction parallel to the +y-axis. In various embodiments theinput interface may comprise an input facet and/or the mode converter301. The external modulated optical signal can have a modulationbandwidth in the range of approximately 5 GHz to approximately 50 GHz.The external modulated optical signal can have an optical power betweenabout −50 dBm and about 10 dBm. An optional optical amplifier (e.g. asemiconductor optical amplifier SOA) 302 can be used to compensate forcoupling losses and passive waveguide loss. In some embodiments, theoptical amplifier can be polarization sensitive and have differentoptical gain for TE and TM modes. The illustrated receiver embodimentfurther comprises a widely tunable local oscillator (LO) laser 303monolithically integrated on the common substrate. The LO 303 mayinclude a tuning section 310. In some embodiments, the LO may beconfigured to emit optical radiation along an optical axis in adirection opposite to the direction of the incoming external opticalsignal (e.g. along a direction parallel to the −y-axis). In someembodiments, the direction of the light emitted from the laser can bechanged by about 180 degrees such that the light emitted from the LO 303propagates in a direction parallel to the direction of propagation ofthe incoming optical signal (e.g. in a direction parallel to the+y-axis). The 180 degree reflection can be achieved by forming a 180degree waveguide turn. In some embodiments the 180 degree waveguide turncan be achieved either by a curved waveguide segment, or by using acombination of waveguides and total internal reflection mirrors 304.

In various embodiments, total internal reflection (TIR) mirrors 304 canalso be integrally disposed on the common substrate. A preferredembodiment of a TIR mirror can comprise a high index-contrastdielectric-semiconductor interface that allows discrete reflection ofthe optical mode between two waveguides. One purpose of these structurescan be to reflect the optical radiation propagating in the waveguide atan angle. In some embodiments, the TIR mirror can comprise at least onereflective facet arranged at an angle θ with respect to the waveguidethat is configured to change the direction of propagation of the opticalradiation by approximately 90 degrees-approximately 180 degrees. TIRmirrors can also be disposed at the input and/or output of opticalcouplers and splitter to allow a rapid transversal displacement of theoptical radiation. This arrangement can be advantageous to achieve acompact fan-out of input or fan-in of output optical waveguides fromoptical splitters and optical couplers in contrast to the more commonlyused S-bends which require a gradual fan-out to maintain low opticalloss. In various embodiments, the use of TIR mirrors can enable areduction in the die size or the footprint of the device since the inputand output waveguides can be fanned-out or fanned-in to achieve thedesired separation between the various sub-components in relatively lessspace. Furthermore, the lengths of optical waveguides can be shortenedin devices using TIR mirrors so as to reduce optical propagation losses.Various embodiments, comprising S-bends to fan-out or fan-in the inputand output waveguides would likely result in an increase in the die sizeor the footprint of the device, since the lengths of the waveguides withS-bends and/or the radius of curvature of the S-bends cannot be reducedbeyond a certain minimum (e.g. in various embodiments, S-bends canexhibit increased loss if the radius of curvature is less than 50microns) without increasing waveguide losses or complicating theintegration platform. Use of TIR mirrors is thus advantageous to realizecomplex devices having reduced die size and footprint by using a simpleintegration platform. Nevertheless, there may be embodiments in whichS-bends, deeply etched waveguide bend or other waveguide structures maybe more preferable than TIR mirrors to achieve fan-out of input orfan-in of output optical waveguides from the optical splitters andoptical couplers.

The illustrated embodiment can further include a 2×N or an M×N typemixer 305 to couple the external modulated optical signal and the laserlight from the LO 303. In various embodiments, the mixer may be disposedat a distance of about 750 μm from the input interface as measured alongthe optical axis. In various embodiments, the optical mixer 305 can bedisposed at a distance of approximately 750 microns or less from theinput interface as measured along a horizontal direction parallel to afirst edge of the die (e.g. along the y-axis). In various embodiments,the mixer 305 can be a multimode interference type. Other types ofmixers such as those described above or otherwise commonly known in theart can be used in some embodiments. The mixer 305 can include at leasttwo output waveguides 306 which are optically coupled to one or morephoto detectors 308. In various embodiments, the mixed output in one ofthe output waveguides can have a relative phase difference φ, betweenthe phase of the external modulated optical signal and the phase of thelocal oscillator LO, while the mixed output in the other outputwaveguide can have a relative phase difference of φ±180 degrees, betweenthe phase of the external modulated optical signal and the phase of thelocal oscillator LO. In some embodiments, one or more trimming pads orelectrodes 307 can be disposed along the two output waveguides 306positioned before the one or more photo detectors 308. Voltage can beprovided to one or both of the electrodes 307 through a drive circuit toadjust the relative absorption in the two output waveguides. In variousembodiments, the applied voltage may apply a reverse bias to theelectrodes 307 and attenuation of the light propagating through thewaveguides 306 can be achieved by utilizing Franz Keldysh or quantumStark effect. The electrodes can generate a weak absorption to attenuatethe optical signal propagating in one or both of the output waveguidesto balance or substantially equalize the optical power input to the twobalanced photo detectors 308.

The one or more photo detectors 308 can be integrated with the commonsubstrate, in the same or different epitaxial structure as the othercomponents. The photo detectors 308 can comprise SiGe, InGaAs, InGaAsP,Si, etc. The photo detectors can have a bandwidth ranging fromapproximately 5 GHz to approximately 50 GHz and be configured togenerate a photo-current in the range of approximately 0.1 mA and 20 mA.In general, the output of the photo detectors 308 can correspond to theamplitude and/or phase of the modulated signal. For example, in someembodiments, the electrical output signal 309 a can correspond to theamplitude and the phase of the external modulated optical signal, whilethe electrical output signal 309 b can correspond to only the amplitudeof the external modulated optical signal. The information related to thephase of the external modulated optical signal can be obtained bysubtracting the output signal 309 b from the output signal 309 a or viceversa. Alternatively, the two photo detectors can be coupled in serieson the die such that the differential component of the output istransmitted out in a single connection. In various embodiments, thephase information obtained can be used to derive a feedback or tuningsignal which can be provided to the tuning section 310 of the LO 303through an external or an on-chip feedback loop. The tuning signal canbe provided to the tuning section 310 in the form of a current or avoltage which can change the optical frequency (or wavelength) of theemitted laser light.

Embodiment 2

FIG. 4 illustrates another embodiment of a coherent receiver device. Asdiscussed above, the illustrated receiver device can be formed on acommon substrate comprising at least one monocrystalline substrate. Thereceiver comprises an input waveguide configured to receive an externalmodulated optical signal, a local oscillator 403, an optional opticalamplifier 404 disposed at the output of the LO 403, TIR mirrors 405, a2×4 or 2×N optical mixer 406 and a plurality of photodetectors/photodiodes 409. In various embodiments, the output waveguidesof the mixer 406 can be rapidly fanned-out by using a plurality of TIRmirrors 407. In various embodiments, the mixed output in one of theoutput waveguides can have a relative phase difference φ, between thephase of the external modulated optical signal and the phase of thelocal oscillator LO, while the mixed output in a second output waveguidecan have a relative phase difference of φ±90 degrees, between the phaseof the external modulated optical signal and the phase of the localoscillator LO. The mixed output a third output waveguides can have arelative phase difference φ±180 degrees, between the phase of theexternal modulated optical signal and the phase of the local oscillatorLO, while the mixed output in a fourth output waveguide can have arelative phase difference of φ±270 degrees, between the phase of theexternal modulated optical signal and the phase of the local oscillatorLO.

In various embodiments, the equivalent structure can be formed by anetwork of 1×2 and 2×2 optical mixers, together with optical phasetrimming pads 408 which in various embodiments can be structurally andfunctional similar to electrodes 307 of FIG. 3. As discussed above,trimming pads 408 can be used to balance the optical power in the fouroutput waveguides of the mixer 406 which are input to the plurality ofphoto detectors 409. The output signals 410 a-410 d from the photodetectors 409 can comprise optical amplitude-plus-in-phase information;optical amplitude-minus-in-phase information;amplitude-plus-quadrature-phase information; and opticalamplitude-minus-quadrature-phase information. Alternatively, byconnecting pairs of photo detectors 409 in series, the in-phase andquadrature-phase information can be transmitted out through two outputsignals. In this arrangement the total optical phase information,including any sine and cosine components can then be recovered from thefour complementary outputs. As discussed above with reference to FIG. 3,the optical phase information can be used to derive a control signalwhich can be provided to the tuning section 411 of the LO 403 by afeedback circuit to change the optical frequency (or wavelength) of theLO 403.

Embodiment 3

The embodiment illustrated in FIG. 5 includes many components andfeatures that are similar to the components and features described inthe embodiment illustrated in FIG. 3. For example, the embodimentillustrated in FIG. 5 comprises a LO 501, an optional optical amplifiersection 502, a mixer 503 and a plurality of photodetectors 506. Invarious embodiments, the embodiment illustrated in FIG. 5 may include amode converter 505 at the input interface. The embodiment illustrated inFIG. 5 further comprises a tunable polarization rotator 504 disposedbetween the output of the LO 501 and the input of the optical mixer 503.The tunable polarization rotator 504 may be similar to the variousembodiments of the polarization rotator described with reference toFIGS. 2A-2C. The output of the tunable polarization rotator can be tunedto match the polarization of the optical signal generated by the LO 501to the polarization of the incoming modulated optical signal coupled into the device through an optical interface. This can be advantageous inefficiently mixing the two optical signals in the optical mixer 503.

Embodiment 4

FIG. 6 illustrates another embodiment of a coherent receiver device. Asdiscussed above, the illustrated receiver device can be formed on acommon substrate comprising at least one monocrystalline substrate. Thereceiver comprises an input waveguide configured to receive an externalmodulated optical signal, an optional optical amplifier 601 that can beused to compensate for coupling losses and passive waveguide loss, anoptical splitter 602 that is configured to split the external modulatedoptical signal between two waveguides 612 a and 612 b, an optionaloptical amplifier 603 disposed in one or both of the output waveguides612 a and 612 b that can be used to compensate for coupling losses andpassive waveguide loss. The device further comprises a widely tunablelocal oscillator 604 comprising a front and a back mirror and a tuningsection 610 that can be integrated on a separate waveguide formed on thecommon substrate. An optional optical amplifier 605 can be disposed atthe output of the laser 604. In the illustrated embodiment, the opticalradiation from the LO laser 604 is split between two output waveguidesof an optical 1×2 mixer structure 606. In various embodiments, apolarization rotator 607 can be integrated with one of the outputwaveguides of the mixer 606. The polarization rotator can be configuredto rotate the optical polarization of the radiation emitted from the LO(e.g. from TE to TM polarization) to match the two possible polarizationstates of the received modulated optical signal. In some embodiments,the polarization rotator 607 can be integrated with either waveguide 612a or waveguide 612 b such that input polarization of the externalmodulated optical signal is rotated instead. The emitted radiation fromthe LO can be mixed with the received modulated optical signal in two2×4 optical mixers 608 a and 608 b. For example, the TE component of thereceived modulated optical signal can be combined with the TE componentof the laser light emitted from the LO in the mixer 608 a, while the TMcomponent of the received modulated optical signal can be combined withthe TM component of the laser light emitted from the LO in the mixer 608b.

Following similar balancing and detection techniques as described above,the output 609 from each of the eight photo detectors 611 can be used toobtain the full phase information for each of the received TE and TMpolarizations. As discussed above with reference to FIG. 3, the opticalphase information can be used to derive a control signal which can beused to change the optical frequency (or wavelength) of the LO.

Embodiment 5

FIG. 7 illustrates another embodiment of a coherent receiver. Asdiscussed above, the illustrated receiver device can be formed on acommon substrate comprising at least one monocrystalline substrate. Thereceiver comprises an input waveguide including a mode converter 701configured to receive an external modulated optical signal, an optionaloptical amplifier 702 that can be used to compensate for coupling lossesand passive waveguide loss, and an optical splitter 703 that isconfigured to split the external modulated optical signal between twowaveguides. An optical amplifier 704 can be integrated with one of theoutput waveguides of the coupler 703. The device further comprises awidely tunable local oscillator 705 comprising a front and a back mirrorand a tuning section 713 that can be integrated on a separate waveguideformed on the common substrate. In some embodiments, an optional opticalamplifier 706 can be disposed at the output of the laser 705. In theillustrated embodiment, the optical radiation the LO laser 705 is splitbetween two output waveguides of an optical 1×2 mixer structure 707. Afirst part of the received modulated optical signal and a first part ofthe LO laser signal is mixed in a 2×4 optical mixer structure 708. Eachof the four optical outputs from the mixer is detected by fourphotodiodes 709. The detected photocurrent from the photodiodes isprovided to an optical modulator 710. In some embodiments, the opticalmodulator 710 is configured to receive only the second part of the LOlaser signal. In some embodiments, the optical modulator 710 can beconfigured to receive both, the second part of the LO laser signal andthe second part of the received modulated optical signal.

The output of the optical modulator 710 can be coupled into a 2×4optical mixer structure 711. Each of the four optical outputs from themixer 711 can be detected by four photodiodes 712. The detectedphotocurrent from the photodiodes can be used to generate a controlsignal that can be applied to the tuning section 713 in the LO laser705. In those embodiments, where the received modulated optical signalis QPSK modulated, the first detector array 709 can detect the in-phaseand quadrature data. The photocurrent from the detector array 709 canthen modulate the input signal in a manner such that the QPSK modulationis cancelled out and the optical carrier is recovered.

In various embodiments, the various integrated optical receiverarchitectures and components can be monolithically integrated on acommon substrate with the various integrated transmitter architecturesand components such as those described in U.S. Provisional App. No.61/182,017 filed on May 28, 2009 titled “Chip-Based Advanced ModulationFormat Transmitter,” which is hereby expressly incorporated herein byreference in its entirety.

While the foregoing detailed description discloses several embodimentsof the present invention, it should be understood that this disclosureis illustrative only and is not limiting of the present invention. Itshould be appreciated that the specific configurations and operationsdisclosed can differ from those described above, and that the apparatusand methods described herein can be used in contexts. Additionally,components can be added, removed, and/or rearranged. Additionally,processing steps may be added, removed, or reordered. A wide variety ofdesigns and approaches are possible.

The examples described above are merely exemplary and those skilled inthe art may now make numerous uses of, and departures from, theabove-described examples without departing from the inventive conceptsdisclosed herein. Various modifications to these examples may be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other examples, without departing from thespirit or scope of the novel aspects described herein. Thus, the scopeof the disclosure is not intended to be limited to the examples shownherein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein. The word “exemplary” isused exclusively herein to mean “serving as an example, instance, orillustration.” Any example described herein as “exemplary” is notnecessarily to be construed as preferred or advantageous over otherexamples.

What is claimed is:
 1. A monolithically integrated optical receivercomprising: at least one monocrystalline substrate; a tunable laserresonator monolithically integrated with the substrate; a first opticalmixer monolithically integrated with the substrate, the first opticalmixer having a first and a second input waveguide, a plurality of outputwaveguides and at least one turning mirror disposed with respect to oneof the input or the output waveguides, the first input waveguide of thefirst optical mixer optically connected to the laser resonator, thesecond input waveguide of the first optical mixer configured to receivea modulated optical signal; a second optical mixer monolithicallyintegrated with the substrate, the second optical mixer having a firstand a second input waveguide, a plurality of output waveguides and atleast one turning mirror disposed with respect to the input or theoutput waveguides, the first input waveguide of the second optical mixeroptically connected to the laser resonator, the second input waveguideof the second optical mixer configured to receive the modulated opticalsignal from the input interface; a polarization rotator monolithicallyintegrated with the substrate; and a plurality of photodetectorsmonolithically integrated with the substrate, each of the plurality ofphotodetectors being optically connected to one of the plurality ofoutput waveguides of the first or the second optical mixer.
 2. Theoptical receiver of claim 1, wherein the substrate comprises at leastone of Si, InP, InAlGaAs, InGaAsP, InGaP, GaAs or InGaAs.
 3. The opticalreceiver of claim 1, wherein the turning mirror has at least onereflective facet that is arranged at an angle with respect to one ofinput or the output waveguides.
 4. The optical receiver of claim 1,wherein the turning mirror is configured to change the direction ofpropagation of optical radiation in one of the input or the outputwaveguides by an angle.
 5. The optical receiver of claim 4, wherein theangle is between approximately 90 degrees and approximately 180 degrees.6. The optical receiver of claim 1, wherein the turning mirror includesa dielectric.
 7. The optical receiver of claim 1, wherein the firstoptical mixer has at least four output waveguides.
 8. The opticalreceiver of claim 7, wherein the second optical mixer has at least fouroutput waveguides.
 9. The optical receiver of claim 1, wherein thepolarization rotator is disposed between the laser resonator and thefirst or second optical mixer.
 10. The optical receiver of claim 10,wherein the polarization rotator includes an input waveguide, an outputwaveguide and at least one electrode disposed with respect to the inputor the output waveguide.